The field of the disclosure relates generally to gas turbine engines and, more particularly, to a method and system for reducing the effects of a bowed rotor during startup of the gas turbine engine and increasing rotordynamic stability against Alford whirl forces.
Gas turbine engines retain an amount of heat after a shutdown. The heat is slowly dissipated over time after the shutdown. During this dissipation period the heat tends to rise in the engine preferentially heating the upper portions of the interior engine components. The temperature gradient created by the rising heat causes the rotor to bow. For example, with the upper half of the rotor at a higher temperature than the lower half of the rotor, the rotor will tend to bow because of differential expansion of the upper and lower halves of the rotor. During a subsequent startup of the engine, the bow can cause a rotor imbalance and associated vibration. Typically, the engine is allowed to idle for a period of time during startup to even the temperatures about the rotor, which permits the rotor bow to be mitigated. However, gas turbine engines sometimes experience a resonant vibratory response to rotor bow at or below idling rotational speeds.
During operation at high torque conditions, a gas turbine can experience a phenomenon called Alford whirl due to tangential aerodynamic forces on the rotor blades. Alford whirl is a well-known phenomenon in the art of rotordynamics. Without sufficient damping of the rotor shaft, the rotor shaft can vibrate in a whirling motion, which may become violent depending on several parameters. A common approach to mitigating Alford whirl is to add damping to the rotor main engine support bearings. In some instances, the damping provided at the bearings is not sufficient to prevent Alford whirl.